Recent increases in network transmission speeds has necessitated the revision of local area network (LAN) and metropolitan area network (MAN) architectures and protocols. A MAN is a network that interconnects users with computer resources in a geographic area or region larger than that covered by even a large LAN but smaller than the area covered by a wide area network (WAN). One of the most popular architectures in MAN's is the bi-directional ring. The bi-directional ring is a simple architecture having two uni-directional, counter-rotating rings. The ring architecture provides fast and easy recovery from fiber-cut and node failures by wrapping the ring near the failed fiber or node. Other LAN and MAN architectures utilize different types of distributed network architectures.
Channel-access mechanisms for distributed networks, such as ring networks, have been studied extensively in the past. For example, in Token Ring networks, a bit- or token-passing scheme is used to prevent the collision of data between two network nodes that want to send messages at the same time. During operation, empty information packets are continuously circulated on the ring network. When a computer node in the ring has a message to send, it inserts a token set equal to 1 in an empty packet and inserts a message and a destination identifier in the packet. The packet is sent onward and then examined by each successive node in the network. When an intermediate node receives the packet, it routes the packet onward after determining that it is not the intended recipient of the message included within the packet. The intermediate note is not able, however, to add a new message of its own to the packet. When the recipient node receives the packet, it copies the message from the packet and changes the token back to 0. When the packet returns to the originating node, it sees that the token has been changed to 0 and removes the message from the packet.
Token Ring protocols have been shown to be effective in certain scenarios. However, as the transmission speed increases, the achievable throughput of the network may saturate at the peak transmission speed of a particular node in the network. In addition, channel-access mechanisms based on a single token, such as the Token Ring protocol, require complex mechanisms to account for lost or duplicate tokens. Multiple tokens can be employed to minimize delay at low loads and improve efficiency at high loads, but the access control scheme may become complex as transmissions from different nodes interfere with each other.
Other access mechanisms and protocols have also been used within distributed network architectures. For example, modern MAN's are predominantly based on the Synchronous Optical Network (SONET) architecture. SONET has been designed to operate in a circuit-switched manner by employing bandwidth sharing in the time domain. However, the share of the bandwidth allocated for a particular node may often be predetermined, which reduces the flexibility within the network architecture.